At service stations and other fueling facilities, it is common to place fuel storage tanks in underground locations with the storage tank inlet stand pipes extending upwardly to just below ground level. The input end of such stand pipes normally is surrounded by a fillbox which is, in essence, an open ended cube or cylinder having an upwardly facing lip for receiving a cover. Unfortunately, because of the heavy vehicle traffic over the cover, it is difficult or impossible to provide an economical seal between the cover and the fillbox. Therefore, rain water, wash water or other fluid contaminants in the area can flow into the fillbox. If the quantity of the fluid is sufficient, its fluid level can extend above the input in the fillbox for a substantial period even though the fluid is draining out through the earth at the open bottom of the fillbox. If the inlet is then opened, the water runs down the stand pipe into the storage tank where it gravitates to the bottom of the tank. Since in conventional present-day arrangements, fuel is pumped from the bottom of the tank, soon thereafter the water is introduced into vehicle fuel tanks causing sputtering, stalling, and starting problems to the vehicle owners as well as cheating them by selling them water instead of fuel. To reduce this problem, the oil company supplying the fuel, must provide maintenance personnel and equipment to pump the fluid out of the fillboxs at considerable expense, and from tanks.
Another solution is to place the inlet above ground. However, this makes gravity transfer of fuel inconvenient. It also is desirable that the inlet be placed in a fillbox and be located slightly underground so that errant vehicles cannot strike it to cause structural damage to the underground tanks or fire.
Therefore there has been a need for a fillbox cover which prevents the entry of fluid contaminants into the fillbox in sufficient quantity to cause overflow of the inlet and the introduction of water into the underground storage tank when the inlet is opened.